The term "bus" is used herein not only for buses proper, but also for any similar urban surface public transport vehicle which is subject to the hazards of urban road travel in the same manner as private vehicles.
Amongst portable appliances of the above-mentioned type, the invention relates more particularly to those which comprise:
radio receiver means for receiving by radio data enabling the above-mentioned waiting times to be known;
interrogation means actuatable by a user to access waiting time information to inform said user on at least the waiting time for a certain bus at the stop under consideration on the route under consideration, said waiting time information corresponding to a certain estimated time of arrival for the bus under consideration at the stop under consideration; and
interface means controlled by an electronic central unit for communicating said waiting time information to the user.
Document WO-A-94/02923 describes an example of such a portable appliance.
That portable appliance operates in satisfactory manner so long as bus drivers continue to drive in a consistent, regular manner, being influenced solely by traffic conditions.
However, from time to time a bus driver who was initially driving in a calm and prudent manner may suddenly take to driving quickly, or conversely a driver who was originally driving quickly may suddenly take to driving slowly, and this can happen independently of traffic conditions.
Such sudden and unforeseeable changes in the behavior of a bus driver can make earlier estimates of the portable appliance concerning the time of arrival of the bus at a given stop quite irrelevant.
Thus, when a bus driver changes suddenly from driving slowly to driving quickly, the bus will reach the next stop sooner than the time of arrival that could be estimated while the driver was driving slowly.
A user who consulted the portable appliance while the driver was driving slowly can thus reach the bus stop at the initially estimated time of arrival, i.e. after the bus has already arrived, thereby running the risk of the user missing the bus.
Conversely, if a driver has switched suddenly from driving quickly to driving slowly, then the bus will arrive at the next stop later than the time of arrival that was initially estimated while the driver was driving quickly.
In this way, a user who has consulted the portable appliance while a driver was driving quickly will arrive at the bus stop at the initially estimated time of arrival for the bus, and will then have to wait unexpectedly for the bus.